The present invention relates to a heat pump and air conditioning system incorporating a thermal storage device. More particularly, the present invention relates to various refrigerant-based heat pump and air conditioning systems incorporating direct expansion thermal storage devices, some of which are suited to contain both encapsulated and unencapsulated phase change materials.
Air source heat pumps extract heat from outdoor air and deliver it to the air distribution system of an indoor space to be heated. In effect, air source heat pumps "pump" heat into a space just as typical air conditioners "pump" heat out of a space.
It is widely recognized, however, that when ambient temperatures fall below a certain limiting level, heat pump efficiency decreases dramatically. That is, a balance point temperature may be defined for heat pump systems at which the heat pump capacity equals the heat loss from the home. Supplemental heating will be required to maintain temperatures in the heated space when the ambient temperature falls below the balance point.
Unfortunately, the balance point for most heat pump systems ranges from about 20.degree. to about 32.degree. F. (about -7.degree. to about 0.degree. C.). Thus, heat pumps operating in typical North American wintertime conditions normally must be provided with supplemental heating.
In addition, heat pumps are often called upon to operate under rapidly changing ambient conditions which may give rise to a mismatch between heat pump heat production capability and heat demand. For example, in operation during a typical winter day, average ambient temperatures may well remain close to the system balance point temperature during the daytime, but may rapidly fall well below the system balance point temperature at night. Thus, the system is likely to operate with excess heating capacity during the daytime and inadequate heating capacity at nighttime. Supplemental heating will likely be required at nighttime.
An analogous phenomenon occurs when the heat pump system is operating in a cooling mode to extract heat from the conditioned space. The efficiency of the heat pump decreases as ambient temperature increases. In typical summertime operation, the heat pump may operate with adequate cooling capacity during daytime hours but will have excess cooling capacity during nighttime hours.
The requirement for supplemental heating reduces any economic benefit that a heat pump system might otherwise provide over conventional heating systems. Moreover, such a system will most probably be operating at highest capacity (and lowest efficiency) during on-peak billing hours (for example, during the daytime generally).
Some researchers have attempted to overcome these problems by incorporating a thermal storage device into the heat pump system. See, for example, U.S. Pat. Nos. 4,100,092; 4,256,475; 4,693,089; 4,739,624; and 4,893,476. Such devices typically use a phase change material to enable thermal energy storage in the form of latent heat as the material changes phase, typically between solid and liquid. The thermal energy storage device would, for example, store the excess heating capacity during daytime winter operation for release during nighttime operation when supplemental heating would otherwise be needed. Analogously, the thermal energy device would store "coolness" during nighttime summer operation and would release the "coolness" during daytime operation, reducing the system power requirements.
Typically, heat pump and air conditioning systems incorporating thermal storage devices have sought to achieve energy savings by reducing the load on the system compressor, or by shifting electrical use patterns by "decoupling" compressor operation from building loads, as in the case of so-called "refrigeration coupled thermal energy storage" systems. Some systems, in fact, are designed to interrupt operation of the compressor altogether at certain times, thereby reducing the overall compressor energy consumption. However, such systems require a supplemental fan to achieve heat transfer directly from the thermal storage medium. Other such systems rely upon existing fans but require substantial additional ductwork to deliver air flow from the fans to the thermal storage device.
In addition, attempts have been made to provide a thermal storage device to provide heat transfer between a working fluid and phase change materials contained in the thermal storage device. Researchers have attempted to encapsulate phase change materials in an effort to maximize surface area available for heat transfer contact with the working fluid. In addition, researchers have developed a variety of phase change compositions suitable for use over various temperature ranges, increasing system flexibility. Examples of designs of thermal storage devices are numerous in the art. See, for example, U.S. Pat. Nos. 3,960,207; 4,127,161; 4,219,072; 4,256,475; 4,283,925; 4,332,290; 4,609,036; 4,709,750; 4,753,080; 4,807,696; 4,924,935; and 5,000,252.
Further, researchers have proposed a variety of control strategies for enhancing operating efficiency of heat pump systems incorporating thermal storage devices. Such control strategies, for example, may involve continuous computation of thermal storage target conditions based upon time, ambient conditions, and/or conditions in the thermal storage device. See, for example, U.S. Pat. Nos. 4,645,908; 4,685,307; and 4,940,079.
These attempts, while numerous, have not heretofore resulted in the widespread adoption of thermal storage devices for use in connection with heat pump systems. A need exists for heat pump systems which can be readily retrofit in existing heat pump systems and which provide a variety of configurations for controlling flow of the working fluid (for example, refrigerant) in a circuit designed to maximize system efficiency and flexibility.
According to the present invention, a heat pump and air conditioning system is provided. The system is operable in at least one of a heating mode and a cooling mode, both modes including a thermal charging cycle and a thermal discharging cycle. The system comprises a refrigerant circuit including a compressor and, in serial connection, a first heat exchanger, an expansion device, and a second heat exchanger. The system further comprises a thermal storage device, first means for connecting the thermal storage device in parallel with the first heat exchanger, a first pair of three-way valves positioned to block flow to and from the first connecting means, second means for connecting the thermal storage device in parallel with the second heat exchanger, and a second pair of three-way valves positioned to block flow to and from the second connecting means. The system further comprises means for controlling the first and second pairs of three-way valves so that during operation in the heating mode, charging cycle, refrigerant from the refrigerant circuit flows in the first connecting means through the thermal storage device, and during operation in the cooling mode, discharging cycle, refrigerant from the refrigerant circuit flows in the second connecting means through the thermal storage device.
Further in accordance with the present invention, a heat pump and air conditioning system is provided. The system is operable in at least one of a heating and a cooling mode, both modes including thermal charging and discharging cycles. The system comprises a refrigerant circuit, a phase change heat exchanger or thermal storage device positioned in the refrigerant circuit, a pair of bypass conduits, and a controller for controlling flow through the bypass conduits. The refrigerant circuit includes a compressor, and, in serial connection, a first heat exchanger, a first expansion device, a second expansion device, and a second heat exchanger. The thermal storage device is positioned in the refrigerant circuit between the first and second expansion devices. The first bypass conduit bypasses the first expansion device, and includes a first controlled valve, while the second bypass conduit bypasses the second expansion device and includes a second controlled valve. The means for controlling operation of the first and second controlled valves operates so that during thermal charging cycle, refrigerant flowing in the refrigerant circuit bypasses the first expansion device and during the thermal discharging cycle, refrigerant bypasses the second expansion device.
In accordance with another aspect of the invention, the first bypass line further bypasses the first heat exchanger and the second bypass line further bypasses the second heat exchanger.
According to yet a further aspect of the invention, a heat pump and air conditioning system operable in at least one of a heating and a cooling mode comprises a refrigerant circuit including a compressor, and, in serial connection, a first heat exchanger, a four-way valve, and a second heat exchanger. The system further includes a thermal storage circuit including a thermal storage device, an expansion device, a first conduit extending between the four-way valve and the expansion device, and a second conduit extending between the four-way valve and the thermal storage device. The system further includes means for controlling operation of the four-way valve so that during operation in the heating mode, charging cycle, and the cooling mode, discharging cycle, refrigerant flowing in the refrigerant circuit flows through the thermal storage device prior to passing through the expansion device, and during operation in the heating mode, discharging cycle and the cooling mode, charging cycle, refrigerant flowing in the refrigerant circuit flows through the expansion device before flowing through the thermal storage device.
In accordance with yet another aspect of the invention, the system further comprises a first bypass conduit extending between the refrigerant circuit and the thermal storage circuit to bypass the first heat exchanger and a second bypass conduit extending between the refrigerant circuit and the thermal storage circuit to bypass the second heat exchanger, and wherein the control means includes first means for directing flow between the refrigerant circuit and the first bypass conduit and second means for directing flow between the refrigerant circuit and the second bypass conduit.
Further in accordance with the present invention, a method is provided for conditioning a space using a heat pump and air conditioning system. The system includes a refrigerant circuit and a thermal storage device and the refrigerant circuit includes a compressor, a four-way reversing valve, and, in serial connection, a first heat exchanger, an expansion device, and a second heat exchanger. The thermal storage device is connected in parallel with both the first and second heat exchangers. The method comprises splitting refrigerant flow from the compressor into a first and a second portion, simultaneously flowing the first portion through the first heat exchanger and the second portion through the thermal storage device.
Advantageously, systems of the present invention regulate refrigerant flow through the first and second heat exchangers to achieve energy savings. In the present systems, in contrast to those of the prior art, compressor operation is continuous. Systems of the present invention therefore avoid the need for supplemental fans directed through the phase change storage medium or supplemental ductwork from existing fans. Thus, systems of the present invention are easier to retrofit with existing heat pump systems currently operating in many settings without the benefit of thermal storage capability. Moreover, systems of the present invention may have higher efficiency in the heating mode as compared to conventional systems due to the reliance on thermal storage. Indeed, systems of the present invention require compressors having smaller compressor ratios than those commonly used in conventional systems, such that reliance on the present systems may allow a single stage compressor to be substituted for a two-stage compressor.
In addition, systems of the present invention rely upon a single refrigerant circuit (including a single compressor) for operation in both heating and cooling modes. Furthermore, no supplemental phase change material for cool storage is necessary with systems of the present invention.
In accordance with yet a further aspect of the invention, the phase change heat exchanger or thermal storage device includes a container defining an interior region configured to receive a first phase change material therein, the first phase change material having a first melt temperature. The thermal storage device further includes at least one refrigerant coil extending through the interior region to deliver a flow of refrigerant therethrough. The device also includes a plurality of phase change capsules disposed in the interior region, the phase change capsules each containing a second phase change material having a second melt temperature higher than the first melt temperature.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.